In conventional manufacturing methods for an optical sheet, a sheet resin material extruded from an extrusion die is passed under pressure through a gap between a pair of engraved roll and mirror-finished metal roll to manufacture a resin sheet. With such a manufacturing method, it is necessary to use a rigid roll with the pressing force allowing some uniform sheet thickness to manufacture a sheet with any optical performance. It is also necessary to transfer a reverse pattern engraved on a mold roll onto a sheet to create the smooth surface or particular surface shape as is necessary for exhibiting an optical performance, at some shaping rate.
In recent years, there is a growing demand for thinner optical sheets in response to needs to downsize the lens to shorten a focal distance for high definition (fine pitch), or needs to reduce a size and weight of an optical sheet itself.
However, with the manufacturing method based on the extrusion molding, it is difficult to produce a thin sheet having such a shape that gives an optical performance sufficient for optical use, with a high thickness accuracy. This is because the contact between a roll and a resin gets worse owing to an unbalanced relation between a resin pressure and a pressing force/rigidity of a roll, which causes the roll to deform on the thin sheet requiring a high thickness accuracy, and the deformed roll cannot press the central portion of the sheet, making it impossible to attain such a thickness distribution that realizes the uniform optical performance.
Meanwhile, as another conventional method of manufacturing a thin sheet, there is a method of manufacturing a resin sheet using a rubber or metal-made elastic roll with a smooth surface as a touch roll (see Japanese Unexamined Patent Publication Nos. 2002-36332 and 2002-36333). However, the metal-made elastic roll with a smooth surface is exclusively applicable to a smooth surface sheet, and even when creating projections or depressions on the sheet, the roll transfers the embossed surface to the sheet at most. To that end, the roll hardly produces an optical functional sheet with a shaping rate and transfer accuracy high enough for optical usage.
Further, there have not been made extensive efforts to transfer a desired lens shape with the method of manufacturing a smooth sheet using this elastic roll based on the common belief that the resin is cooled and solidified on contact with a mold roll, and sufficient transferability cannot be achieved unless the resin is applied with enough pressure upon molding. In particular, conventional manufacturing methods for an optical sheet hardly produce an optical sheet satisfying all of conditions such as the thickness of 300 μm or smaller, a pitch of 300 μm or smaller, and the mold height of 30 μm or larger.
With the aforementioned manufacturing methods for an optical sheet, the shape that gives an optical performance can be transferred with the use of a rigid roll, but a thin film of a uniform thickness distribution is difficult to form with a high thickness accuracy. A thin sheet of a uniform thickness distribution can be produced with a high thickness accuracy by use of an elastic roll, but it is impossible to attain shaping/transferring performances enough for optical use. As a result, the high thickness accuracy for the optical sheet and the high shaping/transferring performances could not be both fulfilled at the same time.
In general, a rear projection screen used for, e.g., a rear projection television is a laminate of two lens sheets. One of the lens sheets, a Fresnel lens sheet, is arranged on a light source side and functions to focus image light from a CRT light source or image light having passed through liquid crystal so as to fall within a predetermined angular range. The other lens sheet, a lenticular lens sheet, is arranged on an observer side, and functions to diverge the image light having passed through the Fresnel lens sheet to an appropriate angular range.
A lens sheet of a fine pitch is required especially for a high definition and high quality rear projection type liquid crystal projection television. Such a lens sheet structure is disclosed in, for example, Japanese Unexamined Patent Publication No. 09-120101. FIG. 8 shows the structure of the lens sheet disclosed in Japanese Unexamined Patent Publication No. 09-120101. As shown in FIG. 8, the lens sheet 101 includes a lenticular lens sheet 102, an external light absorbing layer 103, a diffusion layer 104, and a transparent resin film 105.
The lenticular lens sheet 102 is composed of lens portions 1021 and a transparent supporting member 1022. In general, the lens portions 1021 are formed on the transparent supporting member 1022 with photo-setting resin (hereinafter referred to as “2P resin”).
The external light absorbing layer 103 is arranged on a light exit surface side of the lenticular lens sheet 102 in a position at which the lenticular lens 1021 does not focus the light, that is, a non-light-passing position. The provision of the external light absorbing layer 103 makes it possible to reduce return light back to the observer side, which is a part of the external light incident on the lenticular lens sheet 102 reflected by the light exit surface of the lenticular lens sheet 102, resulting in improvement of an image contrast.
The external light absorbing layer 103 is formed by forming a photosensitive layer on a flat portion of the lenticular lens sheet 102 and then bonding a transfer film coated with a black coating material onto the photosensitive layer, and transferring the black coating material to a portion of the photosensitive layer where the diffusion layer 104 is to be formed (see Japanese Unexamined Patent Publication No. 2001-113538).
Further, the diffusion layer 104 is formed on the light exit surface side of the lenticular lens sheet 102. In the lens sheet 101, while the diffusion of a lens of incidence mainly defines the viewing angle in a horizontal direction, the diffusion layer 104 enables the light diffusion in a vertical direction. In addition, the lenticular lens sheet 2 is laminated with the transparent resin film 105 called a front plate via the diffusion layer 104. The transparent film 105 protects the lenticular lens sheet 102, and is arranged for the purpose of attaining surface glossiness equivalent to that of a general cathode-ray tube television.
Besides, although not shown in FIG. 8, a Fresnel lens sheet is generally provided on a light incident surface side of the lenticular lens sheet 102. This Fresnel lens sheet is made up of a sheet on a light exit surface of which a Fresnel lens is provided. The Fresnel lens is composed of lenses arranged at regular intervals and fine pitches in a concentric form.
In manufacturing the lenticular lens sheet 102 using the 2P resin in the above manner, there arises a problem of an increase in production cost for a rear projection screen due to the expensive 2P resin. There is another problem in that the productive facility for the lenticular lens sheet 102 is complicated because the transparent supporting member 1022 is formed and then, the lens portion 1021 is formed thereon.
Moreover, in the case of using the 2P resin, the lens portion 1021 differs from the transparent supporting member 1022 in material, which leads to poor environmental stability, and causes a lenticular lens sheet 1 to warp. Namely, a resin, which is in a liquid form at a production stage, is cured and completed as a sheet but warps due to a difference in shrinkage factor when cured.
Furthermore, the lens portion 1021 differs from the transparent supporting member 1022 in material, and thus a reflective index varies between the two materials in most cases. That is, the light incident on the lenticular lens sheet 102 enters and refracted in both the lens portion 1021 and the transparent supporting member 1022. The difference in refractive index between them causes color unevenness or limits the transparency in some cases.
Further, since the lens portion 1021 and the transparent supporting member 1022 are different in material, edge blunting easily occurs, and there is a limitation on the shaping performance.
As mentioned above, with the conventional manufacturing methods for an optical sheet, the shape that gives an optical performance can be transferred with the use of a rigid roll, but a thin film of a uniform thickness distribution is difficult to form with a high thickness accuracy. A thin sheet of a uniform thickness distribution can be produced with a high thickness accuracy by use of an elastic roll, but it is impossible to attain shaping/transferring performances enough for optical use. As a result, the high thickness accuracy for the optical sheet and the high shaping/transferring performances could not be both fulfilled at the same time.
The present invention has been made with a view to solving the above-mentioned problems, and therefore it is an object of the present invention to provide a manufacturing method for an optical sheet, which is capable of manufacturing a thin optical sheet of a uniform thickness distribution with a high thickness accuracy and high shape and transfer fidelity, and an optical sheet manufactured using the method.
Besides, the conventional manufacturing method for the lenticular lens sheet has a problem that two different materials are used and therefore, a low-cost, high-quality lenticular lens sheet cannot be formed.
The present invention has been also made in order to solve such a problem, and another object of the invention is to provide a manufacturing method for a lenticular lens sheet, which is capable of manufacturing a low-cost, high-quality lenticular lens sheet, and a lenticular lens sheet manufactured using the method.